The invention relates to monitoring the amount of finely divided material in a fluid sample. More particularly, the invention relates to monitoring the level of particulates in a gas stream, for example, the dust in a smokestack or duct associated with baghouse pollution control equipment. The invention detects backscattered radiant energy as the means for monitoring particulate levels.
Environmental considerations have given rise to regulations requiring the monitoring of particulates in smokestacks and ducts. Initial instrumentation efforts consisted of transmissometers. A transmissometer is a device that projects a beam of light through a particulate-laden gas stream to a photodetector. The transmission of the light is attenuated by the particulates. The degree of attenuation is reported in terms of percent opacity. A number of these devices were developed and installed on smokestacks in the United States and elsewhere. These devices are reliable for the measurement of the opacity of particulates in a gas stream when the opacity exceeds 5%. Below 5% opacity these instruments are not reliable indicators of gas stream particulates because of optical alignment problems and linearity validation problems. Industrial transmissometers are costly, heavy, bulky, and not suitable for the measurement of low dust levels in the small stacks and ducts from industrial air pollution control equipment, particularly the small stacks and ducts associated with baghouse-type pollution control equipment.
Scattering instruments have been developed that measure the presence of particulates by projecting radiant energy into the gas stream and measuring the radiant energy scattered by the particulate. These instruments do not have alignment problems and have a high signal to noise ratio, allowing very low particulate level measurement. These instruments may be side scatter, forward scatter, or backscatter.
Side scatter instruments project a beam of radiant energy into the stack and collect the radiant energy from a section of the beam with a lens that focuses the energy onto a detector. Side scatter instruments have reduced sensitivity to increasing particulate loading due to the opacity of the gas stream between the beam section and the receiving lens. Further, only a small portion of the stack or duct is sampled resulting in incorrect assumptions of particulates loading across the gas stream, unless the particulates are evenly distributed (which rarely occurs).
Forward scatter instruments use a laser beam to project into the gas stream and monitor the forward scattered light from the particulates. These instruments have an advantage in that they can determine the size of a particle from the angle of the energy scattered by the particle. The sample volume is very small reducing this technique""s utility. Because of size, weight and cost considerations, forward scatter instruments are not suitable for monitoring of dust levels in the small stacks and ducts associated with industrial air pollution control equipment.
Back scatter instruments use a projected beam of radiant energy that reflects off particles and is returned to a detector. There is no attenuation of signal as the level of particulate increases as the optical path to any particle reflecting energy will be clear if the projected beam and reflected energy are in the same path. Backscatter instruments usually use a laser to project adequate energy into the duct to assure a return signal. Reflections off the opposing wall are a concern in the measurement process. Many of these instruments are bulky, costly, and heavy rendering them unsuitable for measuring particulate levels in the small stacks and ducts from industrial air pollution control equipment.
The current practice in monitoring dust in the stacks and ducts associated with baghouse-type pollution control equipment is the use of monitors having non-optical probes that extend into the stacks or ducts and generate a measurement of particulate levels by the static charges that incident particles impart to the probe. These devices have the disadvantage of being operative only for use where the particulates being measured hold a charge. Also, these devices have reduced sensitivity and accuracy when the surfaces of the probes become dirty. Probe-type devices as described above are manufactured by Triboflow, Inc. (USA).
There is a need for an optical monitoring system that may be used with relatively small diameter stacks and ducts (for example, less than about five feet diameter) and having particulates therein at opacity levels that are relatively low (for example, less than about five percent opacity) as is the condition in the case of various types of stacks and ducts associated with pollution control equipment, particularly stacks and ducts associated with baghouse-type equipment.
In accordance with the present invention, there is provided a low cost, lightweight solid state backscatter particulate monitor that overcomes the difficulties associated with current transmissometers and scattering instruments. The monitor uses optically efficient design, efficient light-to-digital signal detectors, embedded microcontrollers, an air purge system containing no moving mechanical parts and utilizes less than 2 watts. The monitor provides automatic setup, span and sample operations suitable for measurement of particulates in a gas stream.
In preferred embodiments, the monitor of the invention establishes a transmitted beam of radiant energy into a gas stream that is reflected off particulates in the stream. The monitor includes optics that concentrate the reflected energy onto a detector that generates a signal representative of the amount of energy reflected from the particulates. The radiant energy source, preferably a source of infrared energy, is turned off periodically and the detector output is monitored for ambient energy. A spanning infrared emitting diode (IRED) is provided to periodically illuminate the detector and determine proper instrument operation. An embedded microprocessor sets up the monitor automatically, processes the signal from the detector in setup mode, ambient light mode, signal mode, and span mode. The microprocessor monitors the condition of the instrument, controls the functions of the source, the spanning light emitting diode, and determines the level of the particulates.
In one embodiment, the invention is used as a monitor to determine failure of air pollution control equipment, for example, broken or leaking bags in baghouse pollution control equipment. In this embodiment, the monitor of the invention is mounted to a stack or duct and serves to compare the backscatter level to established levels when the gas stream is clean, determine acceptability to established dust level values, and communicate particulate level conditions to the facility operator. Four conditions are indicated, preferably by using two contacts on the monitor. Both contacts closed indicates that both the monitor and particulate levels are acceptable. If both contacts are open, either the monitor has failed or the power is off. If only one contact is closed, either a mid level warning is indicated or a high level warning is indicated, depending upon the contact.
In another embodiment, the monitor is used as a mass particulate monitor where the microprocessor monitors the condition of the instrument, controls the functions of the source, the spanning light emitting diode, and provides an output of monitor and particulate levels into the facility data acquisition system. Signal output options include, digital, voltage or amperage proportional to the instrument backscatter signal.